SECTION 12
HYDRAULIC STRUCTURES
CITY OF WESTMINSTER
STORM DRAINAGE DESIGN AND TECHNICAL CRITERIA
SECTION 12 HYDRAULIC STRUCTURES
12.1
INTRODUCTION
The energy associated with flowing water has the potential to damage drainage works especially in
the form of erosion. Hydraulic structures are used in storm drainage to control the energy and
minimize the damage potential of runoff. Hydraulic structures include rip-rap, energy dissipaters,
check dams, and drop structures.
The criteria to be used in the design of hydraulic structures shall be in accordance with the
MANUAL except as modified herein.
12.2 RIPRAP
Rip-rap has proven to be an effective means to deter erosion along channel banks, in channel beds,
upstream and downstream of hydraulic structures, and at channel bends. The ordinary rip-rap
classifications and gradations are shown in Table 1201. The design of the rip-rap protection shall
be in accordance with the MANUAL except as modified herein.
12.2.1 Bedding Requirements
The long term stability of ordinary rip-rap erosion protection is strongly influenced by the proper
bedding conditions. The two types of bedding are a granular bedding filter and filter fabric.
For the granular bedding filter, the required bedding thickness as shown in Table 1202 is adequate
for most ordinary rip-rap. The gradation requirements for the granular bedding filter are shown in
Table 1203.
Filter fabric is not a complete substitute for granular bedding. At a minimum, a 4-inch layer of
Type II granular bedding material is required with the filter fabric. Filter fabric’s use is restricted to
slopes no steeper than 2.5 (horizontal) to 1 (vertical). Figure 1201 provides a detail on the typical
lap and filter fabric placement.
12.2.2 Channel Bends
The potential for erosion increases along the outside bank of a channel bend due to the acceleration
of flow velocities. Rip-rap erosion protection is required on channel bends where the radius of the
bend is less than twice the channel top width at the design flow or less than 100-feet. Where
erosion protection is required, Type L rip-rap shall be installed as shown in Figure 1202 and shall
extend upstream and downstream from the bend a distance equal to the length of the bend.
12-1
12.2.3 Conduit Erosion Protection
Erosion resulting from highly turbulent flow at conduit outlets is common. Rip-rap erosion
protection is required at outlets. The following design procedure is valid where the conduit slope is
equal to the channel gradient and the conduit outlet invert is flush with the rip-rap channel
protection. Figure 1203 illustrates rip-rap depth requirements at a conduit outlet.
The required rip-rap classification is selected from Figure 1204 for circular conduits where Q/D2.5
is less than 6.0 and from Figure 1205 for rectangular conduits where Q/WH1.5 is less than 8.0.
The parameters for these two figures are:
Q/D1.5 or Q/WH0.5
Where:
Where:
(12-1)
Q = design discharge, cfs
D = diameter of circular conduit, feet
W = width of rectangular conduit, feet
H = height of rectangular conduit, feet - assumes flow is subcritical
Yt/D or Yt/H
(12-2)
Yt = tailwater depth, feet
D = diameter of circular conduit, feet
H = height of rectangular conduit , feet – assumes flow is subcritical
In cases where Yt is unknown or a hydraulic jump is suspected
downstream, use Yt/D or Yt/H = 0.40
If the flow in the conduit is supercritical at the design flow, then Da is substituted for D and Ha is
substituted H in equation 12-1 and 12-2.
Where:
Where:
Da = 0.5(D + Yn)
D = diameter of circular conduit, feet
Yn = normal depth of flow in conduit, feet
Da shall not exceed D
(12-3)
Ha = 0.5(H + Yn)
H = height of rectangular conduit, feet
Yn = normal depth of flow in conduit, feet
Ha shall not exceed H
(12-4)
The length of erosion protection shall be determined using the method outlined below. However, in
no case shall the length be less than 3D or 3H nor greater than 10D or 10H whenever the Froude
parameter of Q/WH1.5 or Q/D2.5 is less than 8.0 or 6.0 respectively. Whenever the Froude
parameter is greater than these maximums, the maximum length of protection shall increase by onefourth D or H for each whole number the Froude parameter is greater than 8.0 or 6.0 for rectangular
or circular conduits respectively.
12-2
Where:
L = (1/(2tan0- ))(At/Yt - W)
L = length of protection, feet
W = width of conduit, feet
Yt = tailwater depth, feet
1/(2tan0- ) = expansion factor – see Figures 1206 or 1207
(12-3)
At = Q/V
(12-4)
Q = design discharge, cfs
V = allowable non-eroding velocity in the downstream channel, fps
At = required area of flow, ft2
For multiple barrel installations, the methods described can be use by replacing the multiple barrels
with a single hydraulically equivalent rectangular conduit. The dimensions of the equivalent
conduit may be established by distributing the total design flow among the individual barrels and
computing the Froude parameter for each individual barrel. Using the barrel with the greatest
Froude parameter, the height of the equivalent conduit, He , equals the height or diameter of the
selected individual barrel. The width of the equivalent conduit, We , is determined by equating the
Froude parameter from the selected individual barrel with the Froude parameter associated with the
equivalent conduit, Q/WeHe1.5 .
12.3
ENERGY DISSIPATERS
Where rip-rap structures are insufficient or uneconomical to control the erosion potential of storm
runoff, concrete energy dissipater structures (stilling basins) shall be provided in accordance with
the MANUAL.
For culverts or storm sewers outlets where the Froude number at the outlet is in excess of 2.5, the
USBR Type VI impact stilling basin shall be used.
12.4
DROP STRUCTURES
As discussed in the section on open channels, there is a maximum permissible velocity for grasslined channels. One of the more common methods of controlling the flow velocity is to reduce the
channel invert slope. Reducing the channel invert slope requires a drop structure to provide for the
elevation difference.
A drop structure spans across the entire waterway area which conveys the entire major storm flow.
The purpose of a drop structure is to provide a grade drop in both the trickle channel and the main
channels, thus, allowing a milder slope in the channel reach. The geometry of the crest can
effectively control the upstream channel stability and to a great extent its ultimate configuration.
The design criteria for the drop structures shall be in accordance with the MANUAL.
12-3
12.5
CHECK DAMS
A check structure is similar to a drop structure but is constructed to directly stabilize only the bed
and shape of the trickle or low flow channel in natural drainageways. The check dam crosses only a
portion of the channel or floodplain. During a major storm, portions of the flow will circumvent the
check dam. Overall channel stability is maintained because erosion of the low flow channel is
prevented.
The design criteria for check dams shall be in accordance with the MANUAL.
12-4
Table 1201
Classification and Gradation of Ordinary Rip-rap
Rip-rap Designation
% Smaller than Given
Size by Weight
Intermediate Rock
Dimension (in)
Type VL
70-100
50-70
35-50
2-10
70-100
50-70
35-50
2-10
70-100
50-70
35-50
2-10
100
50-70
35-50
2-10
100
50-70
35-50
2-10
12
9
6
2
15
12
9
3
21
18
12
4
30
24
18
6
42
33
24
9
Type L
Type M
Type H
Type VH
1 Mean Particle Size
2 Bury with native top soil and revegetate to protect the rip-rap from vandalism
Specific Gravity of 2.5 or greater.
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-5
d50 1
(in)
6 2
9 2
12
18
24
Table 1202
Thickness Requirement for Granular Bedding
Rip-rap
Designation
VL, L
M
H
VH
Minimum Bedding Thickness
(in)
Fine Grained Soils 1
Course Grained Soils 2
Type I
4
4
4
4
Type II
4
4
6
6
1 May substitute one 12-inch layer of Type II bedding.
2 Fifty percent or more by weight retained on the #40 sieve.
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-6
Type II
6
6
8
8
Table 1203
Gradation for Granular Bedding
U.S. Standard Sieve Size
3”
1-1/2”
¾”
3/8”
#4
#16
#50
#100
#200
Percent Weight By Passing Square Mesh Sieves
Type I
Type II
90-100
20-90
100
95-100
0-20
45-80
10-30
2-10
0-2
0-3
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-7
Figure 1201
Typical Lap Detail and Filter Fabric Placement
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-8
Figure 1202
Erosion Protection for Channel Bends
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-9
Figure 1203
Conduit Outlet Erosion Protection
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-10
Figure 1204
Size of Rip-rap Erosion Protection at
Circular Conduit Outlet
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-11
Figure 1205
Size of Rip-rap Erosion Protection at
Rectangular Conduit Outlet
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-12
Figure 1206
Expansion Factor for Circular Conduits
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-13
Figure 1207
Expansion Factor for Rectangular Conduits
Reference: “Urban Storm Drainage Criteria Manual” DRCOG 1968
12-14
12-15